There is known an image forming device including a photosensitive member cartridge, a toner cartridge, and a developing cartridge, in a removable manner. The photosensitive member cartridge contains a photosensitive member. The toner cartridge contains toner. The developing cartridge contains a developing unit which develops an electrostatic latent image formed on the photosensitive member with the toner contained in the toner cartridge. In the image forming device of this kind, when an electrostatic latent image is formed on the photosensitive member, the electrostatic latent image is developed by the developing unit by using the toner contained in the toner cartridge. Then, the toner is adhered to paper, and thereby the image can be formed on the paper.
Here, in a certain kind of image forming device, for example, life of each of the units is different among each other, i.e. life of the photosensitive member is for 50000 sheets, whereas life of the developing unit is for 20000 sheets and life of the toner cartridge is for 3000 sheets. When the photosensitive member cartridge, the developing cartridge, and the toner cartridge are integrated together and exchanged at the same time, cost effectiveness is lowered. Accordingly, each of the cartridges has suggested to be made separable from the other cartridges so that only the cartridge reaching to the end of life is exchanged at a time (for example, refer to Patent Document 1).    Patent Document 1: Japanese Patent Application Publication No. H04-322260